In any machine or structure there are numerous instances where permanent junctions between component parts are required. The use of welding in assembling metal structures and machines is thus very extensive in present day technology. Typical of such practice is the welding of joints in pipe lines eliminating flanges, couplings and cumbersome fittings. Bridges and buildings of structural steel are fabricated using arc welds to replace riveted connections. Shipbuilders use welding as a principal means of fabrication. Where weight reduction is as important as strength, welding is especially suitable because the weight of rivets and bolts as well as the flanges in which they are seated can be eliminated. To the above illustrations may be added aircraft and missile structures in which welding has now become a standard practice.
As used herein, welding shall mean a method of joining metals by means of fusion or solid state processes. Metals having similar composition may be united in one homogeneous piece by fusing together contacting edges or by adding a molten metal having appropriate metallurgical characteristics at a place where it will form a fused joint with each piece.
Currently accepted weld inspection procedures rely on visual observation and X-ray examination, X-ray inspection is time consuming and cannot provide real-time evaluation of welds being inspected and visual inspection methods fail to detect flawed welds on many occasions because techniques currently in use are very subjective and are highly operator dependent.
In the past, infrared techniques have been used to non-destructively inspect workpieces for internal defects and one such method was described in U.S. Pat. No. 3,504,524. The method described in U.S. Pat. No. 3,504,524 involves spraying the workpiece with a vinyl base carrier of a carbon pigment to form a surface having a constant and high emissivity in the infrared region. After application of the coating, the test surface is heated with a suitable source of radiant energy and the temperatures of successive spots on the test surface are determined by scanning the coated surface with a radiometer. Output from the radiometer is then transmitted to an oscilliscope or other display device where any flaws in the workpiece are displayed as an infrared picture. U.S. Pat. No. 3,504,524 is particularly concerned with controlling the emissivity of the test surface at a standard level by applying a coating to the test surface that has uniform radiating characteristics. An essential characteristic of the coating applied to the test surface is that it must be easily removed when the test is completed.
U.S. Pat. No. 3,020,745 also discloses the use of an infrared detector to test metal objects for flaws. In this method the area being inspected is heated by induced eddy currents which uniformly increase the temperature of the test surface when no flaws are present. If a flaw exists in the test piece the induced current is concentrated about the edges of it and a hot spot develops which is detected by an infrared detector. This method also requires that the test surface be covered with a thin homogeneous coating having a high emissivity. U.S. Pat. Nos. 4,037,473; 3,314,293; and 2,846,882 also disclose the use of infrared detectors to measure the temperature of a workpiece. Additionally, the use of infrared thermography as an inspection tool is discussed in "Trouble-shooting Products through Infrared Thermography" which appeared in the Nov. 10, 1983 issue of Machine Design This paper points out that thermography can be used to characterize a complete temperature field around a particular point of interest. It also points out that only surface temperatures can be measured by this technique and that accurate readings are hard to obtain from shiny surfaces.